Generally, a cathode structure for an electron tube used in a TV system, etc., as shown in FIG. 1, comprises a cylindrical cathode sleeve 2 made of Ni-Cr alloy, a cathode cap 1 made of Ni for sealing the upper end of the cathode sleeve 2, an electron emissive substance 4 deposited on the cathode cap 1, a stepped cylindrical cathode sleeve holder 3 with a small diameter portion and large diameter portion, the small diameter portion being fixed on the lower side of the cathode sleeve 2, and a cylindrical cathode holder 5 for holding a cathode heater 6 and the large diameter portion.
In this cathode structure, the cathode sleeve 2 is blackened by oxidizing the Cr contained in the cathode sleeve in order to improve the heat radiation, thereby reducing the time during which the heater 6 is supplied with a voltage so as to cause a picture signal to appear on the screen of the electron tube. In this case, it should be noted that the cathode cap 1, cathode sleeve 2 and cathode sleeve holder 3 are fixed together prior to the blackening of the cathode sleeve 2 because the blackened cathode sleeve 2 is hardly joined with the cathode cap 1 and cathode sleeve holder 3 by resistance welding. Further, the cathode sleeve holder 3 is not blackened so as to be fixed to the cathode holder 5 by resistance welding.
FIG. 3 is a graph for illustrating the temperature changes of the cathode with variation of the exposed length C of the cathode sleeve in the cathode structure. It will be readily appreciated that the cathode temperature increases with increase of the exposed length C. Consequently, this results in reduction of the power consumption of the heater 6 contained in the cathode sleeve 2. However, the conventional cathode structure inherently limits the exposed length C as well as the reduction of the power consumption.